Magnetic bearing controller

ABSTRACT

A magnetic bearing control device that can operate a motor with low noise, has a simple system construction and is compact and inexpensive comprises: at least a pair of opposed electromagnets for supporting a suspended unit without contact; a motor M for rotating the suspended unit; and a controller for driving and controlling them.  
     A DC/DC converter  13  supplies a driving power to an inverter  14  for driving the motor M. The inverter  14  for driving the motor M uses both PWM and PAM control schemes to drive and rotate the motor M.

TECHNICAL FIELD

[0001] The present invention relates to a magnetic bearing controldevice that controls to suspend a unit without contact by anelectromagnetic force.

BACKGROUND ART

[0002]FIG. 1 shows a construction of a conventional magnetic bearingcontrol device of this type. An AC power from a commercial power supply1 is input via a power factor control circuit (PFC) 2 to a DC/DCconverter 3 for an inverter and a DC/DC converter 4 for the bearing. Anoutput from the DC/DC converter 3 for the inverter is supplied to aninverter 5, and an output from the inverter 5 is supplied to a motor M.An output from the DC/DC converter 4 for the bearing is supplied to adriver 7 for the bearing and a DC/DC converter 8 for the control powersupply. An output from the driver 7 for the bearing is supplied to eachexcitation coil 9 of at least a pair of opposed magnetic bearingelectromagnets (not shown), and an output from the DC/DC converter 8 forthe control power supply supplies an appropriate voltage to anelectronic circuit (not shown) in the device. Thus, a unit to besuspended (not shown) is suspended by an electromagnetic force, androtated by a torque of the motor M.

[0003] In power failures, to use a regenerative power, a power generatedby the motor M controls a frequency of the inverter 5 to control aregenerative voltage to be constant, and is supplied to a backup DC/DCconverter 6. An output from the backup DC/DC converter 6 is supplied tothe DC/DC converter 8 for the control power supply and the driver 7 forthe bearing to decelerate the suspended unit with supported withoutcontact until a safe number of rotation (i.e. a number of rotation thatallows safe support by a touchdown bearing) is reached.

[0004] When the inverter 5 is operated by a PAM (pulse amplitudemodulation) control scheme in the magnetic bearing control device havingthe above described construction, the DC/DC converter 3 for the invertersupplies a gradually increased voltage from a start to the inverter 5,and the voltage is frequency-converted by the inverter 5 and supplied tothe motor M. The PAM control scheme generally produces low noise, buthas a high ripple current resulting in problems in efficiency, a torqueripple, or the like. There is also a problem of a low input power factorat low voltages.

[0005] When the inverter 5 is operated by a PWM (pulse width modulation)control scheme, the DC/DC converter 3 for the inverter supplies aconstant voltage to the inverter 5, and the voltage is pulse-modulatedand frequency-converted by the inverter 5 and supplied to the motor M.The PWM control scheme is generally superior in response in acceleratingand decelerating operations, but inferior in noise of the motor M to thePAM control scheme.

[0006] The magnetic bearing control device having the above-describedconventional construction uses a large number of converters for powersupply, such as the DC/DC converter 3 for the inverter, the DC/DCconverter 4 for the bearing, the backup DC/DC converter 6, and the DC/DCconverter 8 for the control power supply, and, thus, there is also aproblem that the device has a large size and is expensive and complex.

[0007] The present invention has been achieved in view of the above, andan object of the present invention is to provide a magnetic bearingcontrol device that can operate a motor with low noise, has a simplesystem construction and is compact and inexpensive.

DISCLOSURE OF THE INVENTION

[0008] To achieve the object above, the invention according to claim 1provides a magnetic bearing control device including:

[0009] at least a pair of opposed electromagnets for supporting asuspended unit without contact;

[0010] a motor for rotating the suspended unit; and

[0011] an inverter for rotatably driving the motor, the inverterselecting between a PWM control scheme and a PAM control scheme inaccordance with a driving condition of the motor.

[0012] The PWM control scheme is generally superior in response inaccelerating and decelerating operations, but inferior in noise of themotor to the PAM control scheme. The PAM control scheme generallyproduces low noise, but has a high ripple current to cause problems inefficiency, a torque ripple and the like, and cause a low input powerfactor at low voltages. In the invention according to claim 1, asdescribed above, the inverter uses both the PWM control scheme and thePAM control scheme to rotatably drive the motor. Thus, the motor isoperated in the PWM control scheme in the accelerating operation and inthe PAM control scheme in a rated operation, thereby allowing efficientoperation with low noise.

[0013] The invention according to claim 2 provides the magnetic bearingcontrol device according to claim 1, further including:

[0014] a driver (16) for the bearing for driving the electromagnets;

[0015] a DC/DC converter (15) for the bearing for supplying an output tothe driver (16) for the bearing; and

[0016] a DC/DC converter (13) for supplying a driving power to theinverter (14) and the DC/DC converter (15) for the bearing,

[0017] wherein the DC/DC converter (13) outputs a voltage equal to orhigher than a bearing supporting voltage required for supporting thesuspended unit without contact to the DC/DC converter (15) for thebearing, and the DC/DC converter (15) outputs a constant voltage for thebearing supporting.

[0018] The invention according to claim 3 provides the magnetic bearingcontrol device according to claim 1 or 2,

[0019] wherein a change point by which the inverter selects between thePWM control scheme and the PAM control scheme is at least one of a dutyratio in the PWM and an output voltage of the inverter.

[0020] The invention according to claim 4 provides the magnetic bearingcontrol device according to claim 1, further including:

[0021] a driver (16) for the bearing for driving the electromagnets; and

[0022] a DC/DC converter (13) for supplying a driving power to theinverter (14) and the driver (16) for the bearing,

[0023] wherein the DC/DC converter (13) outputs a voltage equal to amagnetic bearing supporting voltage required for supporting thesuspended unit without contact to the driver (16) for the bearing.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024]FIG. 1 shows an example of a construction of a conventionalmagnetic bearing control device;

[0025]FIG. 2 shows an embodiment of a magnetic bearing control deviceaccording to the invention;

[0026]FIG. 3 illustrates the operation of the magnetic bearing controldevice shown in FIG. 2;

[0027]FIG. 4 shows another embodiment of a magnetic bearing controldevice according to the invention; and

[0028]FIG. 5 illustrates the operation of the magnetic bearing controldevice shown in FIG. 4.

BEST MODE FOR CARRYING OUT THE INVENTION

[0029] Embodiments of the invention will be described below withreference to the drawings, exemplifying a case where the invention isapplied to a turbo molecular pump. FIG. 2 shows an embodiment of amagnetic bearing control device according to the invention. An AC powerfrom a commercial power supply 11 is input via a power factor controlcircuit (PFC) 12 to a DC/DC converter 13. An output from the DC/DCconverter 13 is supplied to an inverter 14 and a DC/DC converter 15 forthe bearing. An output from the inverter 14 is supplied to a motor M. Anoutput from the DC/DC converter 15 for the bearing is supplied to adriver 16 for the bearing and a DC/DC converter 17 for the control powersupply. An output from the driver 16 for the bearing is supplied to anexcitation coil 18 of a magnetic bearing electromagnet (not shown), andan output from the DC/DC converter 17 for the control power supplysupplies an appropriate voltage to an electronic circuit (not shown) inthe device. Thus, the unit to be suspended (not shown) is suspendedwithout contact by an electromagnetic force and rotated by the motor M.

[0030] The output from the DC/DC converter 13 is supplied to the DC/DCconverter 15 for the bearing, and the DC/DC converter 15 for the bearingoutputs a constant bearing supporting voltage required for the magneticbearing electromagnet to support the suspended unit without contact tothe driver 16 for the bearing. When a voltage higher than the bearingsupporting voltage is required in accelerating the motor M, the DC/DCconverter 13 further increases its output voltage. However, at thattime, the bearing supporting voltage output from the DC/DC converter 15for the bearing is kept constant. That is, a minimum output voltage ofthe DC/DC converter 13 is a value at which the DC/DC converter 15 forthe bearing can output the bearing supporting voltage required for themagnetic bearing electromagnet to support the suspended unit withoutcontact. The inverter 14 operates in a PWM control scheme in a voltageincreasing operation of the DC/DC converter 15 for the bearing (T1 to T2in FIG. 3) or part of a load varying period (T5 to T6 in FIG. 3). Theinverter 14 operates in a PAM control scheme in a voltage decreasingoperation of the DC/DC converter 15 for the bearing (T4 to T5 and T6 toT7 in FIG. 3).

[0031] In power failures, to use a regenerative power, a power generatedby the motor M controls the frequency of the inverter 14 to control aregenerative voltage to be constant, and is thus supplied via theinverter 14 to the DC/DC converter 15 for the bearing. An output fromthe DC/DC converter 15 for the bearing is supplied to the DC/DCconverter 17 for the control power supply and the driver 16 for thebearing so as to decelerate the suspended unit without contact, until asafe number or rotation of the suspended unit is reached.

[0032]FIG. 3 illustrates an operation of the magnetic bearing controldevice shown in FIG. 2, FIG. 3(a) showing a number of rotation of themotor M, FIG. 3(b) showing an output voltage of the inverter 14, andFIG. 3(c) showing the output of the DC/DC converter 13. As shown in FIG.3(c), the DC/DC converter 13 is started at time T1 to output a voltageV1 required for the magnetic bearing electromagnet to support thesuspended unit without contact. Then, the motor M is accelerated, andthe DC/DC converter 15 for the bearing continues outputting the bearingsupporting voltage until the output from the inverter 14 reaches avoltage V1′ (time T1 to T2). When the motor M is further accelerated andthe motor M requires a voltage equal to or higher than the voltage atwhich the magnetic bearing electromagnet supports the suspended unitwithout contact (time T2), the DC/DC converter 13 increases its outputso as to output a voltage V2 required for a rated operation of a pumpload (time T3).

[0033] During period T1 to T2, the output voltage of the inverter 14continues increasing, but the DC/DC converter 15 for the bearing outputsthe constant bearing supporting voltage required for the magneticbearing electromagnet to support the suspended unit without contact. Inthis period, the inverter 14 operates in the PWM control scheme andturns to the PAM control scheme at the time when a duty ratio of the PWMreaches a maximum.

[0034] After the duty ratio of the PWM reaches the maximum (time T2),and in a rated operation period of the pump load (time T3 to T4), theinverter 14 operates in the PAM control scheme and controls the outputvoltage.

[0035] In the load varying operation period (time T4 to T7), theinverter 14 operates in the PAM control scheme in a period when itsoutput voltage is equal to or higher than V1′ (T4 to T5 and T6 to T7)and in the PWM control scheme in a period when its output voltage islower than V1′ (time T5 to T6) so as to control the output voltage.However, in the load varying operation period, the control scheme of theinverter 14 switches from the PWM to the PAM when the duty ratio of thePWM reaches the maximum, and switches from the PAM to the PWM when theduty ratio of the PWM becomes equal to or lower than the maximum.

[0036] In a power failure (time T7 to T8), the motor M is decelerated byregenerative braking, as described above.

[0037]FIG. 4 shows another embodiment of a magnetic bearing controldevice according to the invention. An AC power from a commercial powersupply 11 is input via a power factor contorol circuit (PFC) 12 to aDC/DC converter 13. An output from the DC/DC converter 13 is supplied toan inverter 14, a driver 16 for the bearing and a DC/DC converter 17 forthe control power supply. An output from the inverter 14 is supplied toa motor M. An output from the driver 16 for the bearing is supplied toan excitation coil 18 of a magnetic bearing electromagnet (not shown).An output from the DC/DC converter 17 for the control power supplysupplies an appropriate voltage to an electronic circuit (not shown) inthe device. Thus, a unit to be suspended (not shown) is suspendedwithout contact by an electromagnetic force and rotated by the motor M.

[0038] The DC/DC converter 13 outputs, to the driver 17 for the bearing,a bearing supporting voltage required for the magnetic bearingelectromagnet to support the suspended unit without contact. Theinverter 14 receives the bearing supporting voltage and operates in aPWM control scheme in accelerating the motor M and in a PAM controlscheme in a rated operation of a pump load.

[0039] In power failures, to use a regenerative voltage, a powergenerated by the motor M controls the frequency of the inverter 14 tocontrol the regenerative voltage to be constant, and is thus suppliedvia the inverter 14 to the DC/DC converter 17 for the control powersupply and the driver 16 for the bearing to decelerate the suspendedunit without contact until a safe number of rotation of the suspendedunit is reached.

[0040]FIG. 5 illustrates the operation of the magnetic bearing controldevice shown in FIG. 4, FIG. 5(a) showing a number of rotation of themotor M, FIG. 5(b) showing an output voltage of the inverter 14, andFIG. 5(c) showing the output of the DC/DC converter 13. As shown in FIG.5(c), the DC/DC converter 13 outputs, to the inverter 14, the driver 16for the bearing and the DC/DC converter 17 for the control power supply,a constant voltage V1 required for the magnetic bearing electromagnet tosupport the suspended unit without no contact. In accelerating the motorM (time T1 to T2), the inverter 14 operates in the PWM control schemeuntil its output voltage reaches V1′ (time T2) and controls the outputvoltage. In the rated operation of the pump load (time T2 to T3), theinverter 14 operates in the PAM control scheme and maintains the outputvoltage V1′.

[0041] In a load varying operation (time T3 to T4), the inverter 14operates in the PWM control scheme and controls the output voltage. Inpower failure (time T4 to T5), the motor M is decelerated byregenerative braking.

[0042] As described above, according to the embodiments of the magneticbearing control device of the invention, the inverter 14 operates in thePWM control scheme in the accelerating operation of the motor M and inthe PAM control scheme in the rated operation of the pump load, thusallowing the motor M to operate efficiently with low noise. It is notedthat the present invention is not limited to the above described twoembodiments, but various modifications and changes may be made by thoseskilled in the art without departing from the spirit and scope of theinvention. For example, detection means for detecting a position of thesuspended unit may be provided in both of the embodiments.

INDUSTRIAL APPLICABILITY

[0043] In a magnetic bearing control device according to the invention,as described above in detail, the inverter uses both PWM and PAM controlschemes to drive and rotate the motor. Thus, the motor is operated inthe PWM control scheme in the accelerating operation of the motor and inthe PAM control scheme in the rated operation, thereby allowingefficient operation of the motor with low noise. Therefore, the presentinvention can be applied to vacuum pumps such as turbo molecular pumps.

1. A magnetic bearing control device comprising: at least a pair of opposed electromagnets for supporting a suspended unit without contact; a motor for rotating said suspended unit; and an inverter for driving said motor, said inverter selecting between a PWM control scheme and a PAM control scheme in accordance with a driving condition of said motor.
 2. The magnetic bearing control device according to claim 1, further comprising: a driver (16) for the bearing for driving said electromagnets; a DC/DC converter (15) for the bearing for supplying an output to said driver (16) for the bearing; and a DC/DC converter (13) for supplying a driving power to said inverter (14) and said DC/DC converter (15) for the bearing, wherein said DC/DC converter (13) outputs, to said DC/DC converter (15) for the bearing, a voltage equal to or higher than a bearing supporting voltage required for supporting said suspended unit without contact, and wherein said DC/DC converter (15) outputs a constant voltage for the bearing supporting.
 3. The magnetic bearing control device according to claim 1 or 2, wherein a change point by which said inverter selects between the PWM and PAM control schemes is at least one of a duty ratio in the PWM and an output voltage of said inverter.
 4. The magnetic bearing control device according to claim 1, further comprising: a driver (16) for the bearing for driving said electromagnets; and a DC/DC converter (13) for supplying a driving power to said inverter (14) and said driver (16) for the bearing, wherein said DC/DC converter (13) outputs, to said driver (16) for the bearing, a voltage equal to a magnetic bearing supporting voltage required for supporting said suspended unit without contact. 